The present invention relates to a mobile telecommunication system comprising a number of base stations which can communicate with mobile stations. A communication from a mobile station to a base station is done by means of an up-link UL and the communication from a base station to a mobile station is done by means of a down-link DL.
The present invention relates also to a telecommunication systems wherein different user signals are separated both in time domain and in code domain. An example of such a system is the so called UMTS TDD system (Universal Mobile Telecommunication Systems—Time Division Duplex) or W-CDMA TDD system (Wideband Code Division Multiple Access—Time Division Duplex) in which the time domain is represented by the TDD-system component and the code domain by W-CDMA-system component.
More particularly, in time-domain, transmission is for example organized based on radio frames constituted of a number (for example 15) of timeslots. The same frequency is used for both the up-link (Mobile Station to Base Station) and the down-link (Base Station to Mobile Station). Furthermore, a time-separation is used to differentiate the down-link and the up-link such that a subset of all the available timeslots per frame is exclusively allocated for down-link transmission and the remaining ones for up-link transmission. In a frame, at least one timeslot is always allocated for each down-link and up-link.
In such a system, different user's signals can be transmitted in separate timeslots, e.g. N different down-link timeslots are allocated to N different down-link user signals. This is the time-division aspect of the system. Furthermore, several users signals can also be transmitted within one timeslot by using different spreading codes. This is the code-division aspect of the system. Note that each user is allocated a different spreading code and that each user bit is spread to the chip-rate as function of the employed spreading factor.
In such a system, the network allocates to each cell covered by the base stations different cell parameters which enable any mobile station attempting to establish connection to said base station to read cell broadcast information needed to communicate therewith. These cell parameters indicate for example a midamble number and a scrambling code. The midamble is a complex- or real-valued chip sequence and is used by a receiver (a mobile station in the downlink) for channel estimation which is needed for the retrieval of the user's signals. The scrambling code is used by the transmitter (a base station in the downlink) to scramble the user's signals in order to average the interference caused to user's who are sending or receiving in neighboring cells.
When a mobile station gets switched on, it must first find out chip, slot and frame timing of at least one cell covering the area in which it is and then find out which midamble and scrambling code are used before it can demodulate and read the cell broadcast information. Afterwards, a tracking mechanism ensures that especially chip timing is not lost, once the mobile station is “synchronized” to the cell.
Each base station, for each cell, transmits the cell broadcast information on a channel which is generally the so-called Primary Common Control Physical Channel (P-CCPCH). It can also be the Secondary Common Physical Channel (S-CCPCH) when it is pointed to by the Primary Common Control Physical Channel P-CCPCH.
Note that a Primary Common Control Physical Channel P-CCPCH in the W-CDMA TDD system generally uses a fixed and pre-allocated spreading code with fixed spreading factor, e.g. its spreading code is the same in all cells of the W-CDMA TDD system and therefore always known before-hand by the mobile station.
A physical synchronization channel (PSCH) is also transmitted simultaneously in those timeslots in the downlink where a Primary Common Physical Channel P-CCPCH is transmitted for the purpose of synchronization to the Primary Common Control Physical Channel (P-CCPCH). The Physical Synchronization Channel essentially consists of two special signals: the primary synchronization code PSC and a set of K secondary synchronization codes SSC. The number K of secondary synchronization codes SSC is generally 3. A Primary Common Physical Channel P-CCPCH is never allocated to a particular timeslot when there is no Primary Synchronization Channel PSCH simultaneously present. If the mobile station finds out in which timeslots the Primary Synchronization Channel PSCH is sent, it knows that the Primary Common Physical Channel P-CCPCH is also in this timeslot.
Each of the K in parallel transmitted secondary synchronization code SSC spreads a symbol having a number n of states, i.e. a modulated Quadrature Phase Shift Keying (QPSK) symbol, which gives a total of nK QPSK codewords.
On one hand, the combination of the code sets, e g. different triplets to spread the QPSK-symbols and, on the other hand, the modulation of these QPSK-symbols are used for indicating:                A Code Group for which are defined univocally one or several cell parameters, e g. one or several scrambling codes together with one or several basic short or long midamble codes,        The position of the Primary Synchronization Channel PSCH within a double-frame period, and        The position of the current Primary Synchronization Channel PSCH timeslot within one frame.        
Finally, at power-on, a mobile station first searches by performing a correlating process for the presence of the Primary Synchronization Code PSC transmitted on the Primary Synchronization Channel PSCH by the base station of the cell under the coverage of which it is and uses the found time positions for correlating with all possible secondary synchronization codes SSC, generally 16. By performing a coherent detection, e.g. using the Primary Synchronization Code PSC as a phase reference for the secondary synchronization codes SSC, it can also detect the QPSK-symbols spread by the K detected secondary synchronization codes SSC. From this information it can derive the time position of the Primary Synchronization Channel PSCH slot within the frames period as well as the Code Group to which the base station belongs. In a last step, the mobile station can demodulate a burst on the Primary Common Control Physical Channel P-CCPCH by trying all still possible of Scrambling Codes and Basic Midamble Codes which are contained in the found Cod Group.
Each secondary synchronization code SSC is a different binary-valued chip sequence which is referenced by a particular index. For example, when 16 secondary synchronization codes SSC are possible in the mobile communication system, each secondary synchronization code SSC is indicated by one of the following values:
SSC0, SSC1, SSC2, . . . , SSC15.
For example, each of the secondary synchronization code SSC is formed according to the rules defined in the Technical Specifications 3GPP TSG RAN TS25.213 v320 “Spreading and Modulation (FDD)”, section 5.2.3.1 page 21ff. and 3GPP TSG RAN TS25.223 v320 “Spreading and Modulation (TDD)”, section 7.1 page 10ff.
Not all of the possible and available secondary synchronization codes SSC are used simultaneously in a cell for the synchronization purposes above described Currently, for making the choice of each set of K secondary synchronization codes SSC allocated to a cell, the network takes out of the N possible ones, the first K secondary synchronization codes SSC, then the second K secondary synchronization codes SSC, etc. M out of N are chosen, the last N−M being unused. For the W-CDMA TDD, system, wherein K is 3 and N is 16, for simplicity reasons, the following 4 code groups (e.g. triplets of SSC) are chosen by the network and allocated to the specific cells:
Code group 1: SSC0, SSC1, SSC2 
Code group 2: SSC3, SSC4, SSC5.
Code group 3: SSC6, SSC7, SSC8.
Code group 4: SSC9, SSC10, SSC11.
Thus, only M=K*L secondary synchronization codes SSC out of N possible and available ones are used for synchronization purposes
The performance of the synchronization process above described is very sensible to the errors than can occur during transmission or during the correlation processes performed for the retrievals of the primary synchronization code PSC and of the secondary synchronization codes SSC.